Arterial Blood Gas (ABG) Analysis

Life is a struggle, not against sin, not against the Money Power, not against malicious animal magnetism, but against hydrogen ions. - H.L. MENCKEN

A Guide to Interpretation of Arterial Blood Gases (ABGs)

Arterial blood gases (ABGs) are an important diagnostic tool for the evaluation of patients. They are useful for determining the ventilation status, the oxygenation status, and the acid-base balance of the patient.

Definitions of Acid Base Disorders

Acidosis — A process that tends to lower the extracellular fluid pH (hydrogen ion concentration increases). This can be caused by a fall in the serum bicarbonate (HCO3) concentration and/or an elevation in PCO2.

Alkalosis — A process that tends to raise the extracellular fluid pH (hydrogen ion concentration decreases). This can be caused by an elevation in the serum HCO3 concentration and/or a fall in PCO2.

Metabolic acidosis — A disorder that causes reductions in the serum HCO3 concentration and pH.

Metabolic alkalosis — A disorder that causes elevations in the serum HCO3 concentration and pH.

Simple acid-base disorders — The presence of one of the above four disorders with the appropriate respiratory or renal compensation for that disorder.

Mixed acid-base disorders — The simultaneous presence of more than one acid-base disorder. Mixed acid-base disorders can be suspected from the patient’s history, from a lesser or greater than expected compensatory respiratory or renal response, and from analysis of the serum electrolytes and anion gap. As an example, a patient with severe vomiting would be expected to develop a metabolic alkalosis due to the loss of acidic gastric fluid. If, however, the patient developed hypovolemic shock from the fluid loss, the ensuing lactic acidosis would lower the elevated serum HCO3 possibly to below normal values, resulting in acidemia.